1. Field of the Invention
The present disclosure relates generally to systems, tools and associated methods utilized in conjunction with hydrocarbon recovery wells. More particularly, embodiments of the disclosure relate managing a down-hole pressure differential, which occurs during perforating operations.
2. Background Art
Often, in subterranean wellbores drilled in connection with exploration or the recovery of hydrocarbons, a number of tubular casing members are installed in the wellbore to prevent collapse of the wellbore wall and to manage fluid communication between the wellbore and a surrounding geologic formation. Conventionally, the casing members are cemented within the wellbore, and thus, to pass fluids between an interior of the casing members and the geologic formation, perforations are often formed through the casing members, through the cement and a short distance into the geologic formation. Typically, these perforations are created by detonating a series of explosive shaped charges within the wellbore. In some instances, the shaped charges are loaded into one or more perforating guns at a surface location, and then the perforating guns are lowered down-hole on a conveyance such as a tubing string, wire line, slick line, coil tubing, etc.
The perforating guns may include an outer canister in which the shaped charges are packed. The outer canister can be sealed at the surface location, thereby capturing atmospheric gasses at an ambient surface pressure within a “free volume” defined in the outer canister, e.g., the empty space between the shaped charges. Upon detonation of the shaped charges within the wellbore, detonation gasses fill the canister and the interior pressure may rise to tens of thousands of psi within microseconds. Holes created in the canister by the shaped charges permit the detonation gasses to exit the canister, leaving the free volume in the canister substantially empty. Then the free volume rapidly fills with wellbore fluids and formation fluids.
The transient pressure differential generated as the free volume fills with wellbore fluid may be referred to as a “dynamic underbalance.” In some instances a dynamic underbalance is beneficial. For example, it has been found that the dynamic underbalance may help clean perforation tunnels of debris as formation fluids flow through the tunnels toward the free volume. However, in some instances, an excessive dynamic underbalancemay be detrimental. For example an excessive dynamic underbalance may cause damage to the perforating gun, conveyance tubing, packers set in the wellbore or other down-hole equipment as wellbore fluids rush into the free volume. An excessive dynamic underbalance may also cause damage to the perforation tunnels, e.g., by sanding, as sand is carried into the perforation tunnels by formation fluids rushing through the perforation tunnels toward the free volume. Accordingly, a need has arisen for an apparatus and method that manage a dynamic underbalance to provide for safe and effective perforation of a wellbore.